This invention relates to an amplifier device.
An amplifier device, for example, a cascode amplifier is constituted by a pair of cascode connected transistors in which a transistor of the second stage having a low input impedance with a base electrode grounded acts as a load of a transistor of the first stage having an emitter electrode grounded so that it can provide a stable amplifier action and an improved frequency characteristic. Accordingly, the cascode amplifier is widely used as a high frequency amplifier in a frequency modulated tuner or the like.
As shown in FIG. 1, a prior art cascode amplifier comprises an input terminal adapted to receive an input signal, an output terminal 12, a first stage transistor 14 with its emitter electrode grounded, a second stage transistor 15 with its base electrode grounded, and a bias source 16 having a voltage of E connected between the base electrode of the transistor 15 and the ground. The emitter electrode of the transistor 14 is connected to the ground through an emitter resistor 18, while the collector electrode of transistor 15 is connected to a positive terminal 21 of a source having a voltage of +V via a collector resistor 19 and to the output terminal 12. I.sub.C shows the collector current of the transistor 15. The bias source 16 has a voltage so as to ground the base electrode of the second stage transistor 15 and to provide a sufficiently wide dynamic range at a time of a large amplitude and at a time of the variation in the voltage of the source.
In the cascode amplifier described above an input signal applied to the input terminal 10 is amplified by the first and second transistors 14 and 15 respectively to obtain an amplified output as the output terminal 12.
There is a following relationship between the base-emitter voltage V.sub.BE2 of the transistor 15, the collector-emitter voltage V.sub.CE1 of the transistor 14, the resistance R.sub.E of the emitter resistor 18, the collector current I.sub.C and the source voltage E. EQU e = v.sub.be2 + v.sub.ce1 + i.sub.c r.sub.e ( 1)
thus, EQU V.sub.CE1 = E - V.sub.BE2 - I.sub.C R.sub.E ( 2)
since V.sub.BE2 of transistor 15 and the voltage E of the bias source 16 may be considered substantially constant, the variation .DELTA.V.sub.CE1 of the collector-emitter voltage V.sub.CE1 of the first stage transistor 14 when the collector current I.sub.C varies in accordance with the input signal can be derived from equation (2) as follows. EQU .DELTA.V.sub.CE1 = - .DELTA.I.sub.C .multidot.R.sub.E ( 3)
as can be noted from this equation, in the prior art cascode transistor amplifier, the current feedback voltage provided by the emitter resistor 18 directly causes the variation in the collector-emitter voltage V.sub.CE1 of the first stage transistor 14. This means an increase in the load resistance (the emitter resistor 18 can be regarded as a load resistance) of the first stage transistor. Accordingly, in the prior art cascode amplifier, the deformation of the output signal is large. Further, where a signal having a large amplitude is to be amplified it is necessary to increase the voltage of the source 16 for the following reason. In response to the input signal the emitter potential of the transistor 14 increases, but the collector voltage of transistor 14 must be higher than its emitter voltage. Accordingly, the voltage of the source 16 should be higher than the emitter voltage. For this reason, it is necessary to increase the source voltage E for an input having a large amplitude. However, as the source voltage is increased the collector-emitter voltage V.sub.CE1 of the first stage transistor 14 increases in the absence of the input voltage with the result that the loss (V.sub.CE1 .multidot. I.sub.C) of the transistor 14 increases.